For example, there is a known image sensor consisting of a two-dimensional array of light receiving portions with embedded photodiodes. In each of the embedded photodiodes, for example, an n-type low-concentration semiconductor region is formed on a p-type substrate and a thin p-type high-concentration semiconductor region is formed on a surface of this n-type low-concentration semiconductor region. For readout of charge, an n-type high-concentration semiconductor region is formed on the n-type low-concentration semiconductor region. Since the n-type low-concentration semiconductor region can be completely depleted in this embedded photodiode, charge generated in a pn junction part can be completely read out and occurrence of leak current is suppressed, so as to achieve excellent S/N ratios in detection of light.
In this embedded photodiode, the sensitivity of detection of light can be improved by increasing the area of the photosensitive region or the area of the n-type low-concentration semiconductor region and p-type high-concentration semiconductor region. However, the increase in the area of the n-type low-concentration semiconductor region will lead to incomplete readout of the generated charge, so as to leave a remnant charge in readout. As a result, image lag will occur.
With regard to this problem, the image sensor described in Patent Literature 1 has an impurity concentration slope from an edge of a p-type charge accumulation region 206 (which correspond to the n-type low-concentration semiconductor region being the photosensitive region mentioned above) in the embedded photodiode formed on a n-type substrate 201 instead of the p-type substrate toward a transfer electrode 103 (which correspond to the n-type high-concentration semiconductor region being a transfer electrode for readout of charge mentioned above), so as to lower a potential. Patent Literature 1 describes that this configuration reduces the remnant charge in readout.